1. Field of Invention
The present invention relates to a backlight module, and more particularly, to a backlight module using a light emitting diode for enhancing color saturation of a liquid crystal display (LCD).
2. Related Art
With the tremendous progress of science and technology and high development of network, multimedia technique, image data transmission, the demanding for multimedia displays is greatly increasing. In recent years, LCDs having the advantages of thin thickness, light weight, portability, and low radiation compared with conventional Cathode Ray Tube (CRT) displays have been widely used in consumable electronic industries, such as monitors, notebooks, and digital cameras, and have gradually become the mainstream display products in the market.
Backlight module is one of key parts of an LCD Module. Since liquid crystals do not project lights by themselves, the backlight module has a light guide function for providing evenly-distributed light sources and sufficient brightness, so as to display images normally. However, the requirement on the brightness of LCDs and the color saturation of images grows increasingly.
Light sources used in the backlight module mainly include cold cathode fluorescent lamps (CCFL), hot cathode fluorescent lamps, light emitting diodes (LED), electro-luminescence (EL) elements, and the like. Due to the advantages of small size, low operating current, low power consumption, and long service life, the LEDs will become the main light source applied in the backlight module.
Some techniques of using LEDs as the light sources for the backlight module have been disclosed in prior arts. However, there are still many problems to be solved. In the prior art, three LEDs including red light, blue light, and green light LEDs are used to project red, blue, and green lights which are the three primary colors of light, and the three colors of lights can be mixed into a white light. However, the LEDs of three colors are packaged into an encapsulated housing, and then the ON/OFF and intensity of the three LEDs are controlled respectively, thus causing troubles in control. The three color light sources must used together with the design of light guide portion on the light guide plate, for example, several light guide portions fabricated on the bottom surface of the light guide plate via a screen printing technique. The light guide portions are rough surface design of the bottom surface of the light guide plate. Based on the diffusion principle, the incident light is diffused by the light guide portion, and then passes through the light guide plate surface. However, as different light colors indicate different wavelengths, the light guide plate generates different reflection ratio and diffusion capability to the light rays with different wavelengths. Therefore, if the light guide structure of the light guide plate is designed directed to the brightness uniformity of the red light, it certainly influences the brightness uniformity of the blue light and the green light. Therefore, if the red, blue, and green LEDs are lit up, the lights of three colors cannot be uniformly mixed into the white light.
In order to uniformly blend the light sources of the backlight module into a white light, a backlight module is disclosed in U.S. Patent No. 2006/0072339, entitled “Backlight Module”, which has a blue light LED as a light source and a light guide plate, and a fluorescent substance is placed between the light source and the light guide plate. The fluorescent substance when irradiated by the blue light is activated and projects a yellow light. It is much easier for the lights of two colors i.e. the blue light and the yellow light to be uniformly mixed into a white light as compared with the lights of three colors, thereby forming a plane light source of the backlight module. At this time, since the yellow light is not one of the three primary colors of light, but the mixture of the red light and the green light. Therefore, the color purities of the red light frequency spectrum and the green light frequency spectrum contained in the yellow light frequency spectrum are not high, so it cannot meet the requirements on the color saturation of LCDs.
In order to solve the above problems, a color filter with high purity can be used for the above color lights to pass through, and the color light passing through the high-purity color filter has a relative high color purity of the light frequency spectrum. Relatively, the high-purity color filter filters out a major part of the light rays beyond required frequency spectrums, thus greatly reducing the transmittance of the light rays which causes the problem of insufficient brightness of the LCDs.
Therefore, in the above technologies, in order to produce LCDs with high color saturation and brightness, high-purity color filters and high-brightness LED must be used or the number of LEDs must be increased, which relatively enhances the manufacturing cost of the backlight module. However, in order to solve the problem of the color purity of light sources, a high-purity color light can be used together with the design of light guide portion on the light guide plate, such that the color light can be uniformly mixed into the white light, thereby enhancing the color saturation of LCDs without reducing the brightness of LCDs, which can be used to solve the problems in the prior art.